It is known in the art of polyurethane foam manufacture that a block of freshly made open cell flexible cellular polyurethane foam can be rapidly cooled or cured by subjecting the foam blocks to an air stream alone, or a stream that contains a chemical reactant or that is saturated with water. This rapid cooling has the effect of accelerating and shortening the time for final curing of the foam and produces a block having enhanced physical properties. A further advantage of the rapid cooling process is the prevention of a scorched appearance within the interior of flexible polyurethane foam. Examples of the processes and apparatus for the rapid cooling of foam blocks are disclosed in the following: U.S. Pat. No. 3,061,885 issued Nov. 6, 1962; U.S. Pat. No. 3,890,414 issued Jun. 17, 1975; U.S. Pat. No. 4,537,912 issued Aug. 27, 1985; and application U.S. Ser. No. 07/341,138 filed Apr. 20, 1989, now abandoned in favor of FWC 07/674,438 filed Mar. 22, 1991. These disclosures are herein incorporated by reference.
It has been found that the exhaust air stream from the rapid cooling of freshly-made, uncured flexible polyurethane foam produces a dense white cloud which has the appearance of smoke when discharged into the atmospheric environment. The content of this smoke was found to be very small particles and organic vapors. In operating the rapid cooling process, a means of removing or reducing the particulate matter and vapors discharged into the atmosphere would be desirable.
Other areas of the manufacture or processing of polyurethane foam are known to produce emissions of particulate matter and vapor which are typically carried away from the production area by exhaust air streams for discharge into the atmosphere. For example, as a freshly poured cellular polyurethane foam forming composition completes its rise and assumes its final self-sustaining configuration, it emits a smoke or haze. This smoke is comprised of fine particulate matter and vapors which are emitted as the foam reaches its peak rise and thereafter during the curing period. In order to maintain the quality of the ambient air in the foam production facility, the foam production line is at least partially enclosed along its top and sides and an exhaust air stream is drawn over the reacting foam to collect any reaction byproducts which are emitted. The exhaust air stream is typically discharged through an exhaust duct and roof vent into the atmosphere.
Fine particulate matter and vapors are also generated when polyurethane foam and other foamed and solid plastics are subjected to high temperatures during manufacture and/or fabrication processing. Such processing of polyurethane foam includes heat sealing, hot wire cutting, heat compression, surface melting (such as embossing) and flame lamination. During these methods of processing polyurethane foam, toluene diisocyanate and other gaseous and solid matter are generated which are conventionally drawn away from the foam by an exhaust air stream. It would be desirable to provide an economical and effective means of removing all or a substantial proportion of the fine particulate matter and vapors from the process exhaust air stream before the exhaust air is discharged into the environment.
The fine particles and vapors emitted from the reacting foam range in size from close to zero to about five microns, and include a variety of materials. Analysis of the fine particles and vapors emitted during the rapid cooling of polyurethane foam employing water saturated air, indicates that they are generally insoluble in water, and include impurities, auxiliary blowing agents (such as methylene chloride and fluorocarbons), unreacted excess toluene (or tolylene) diisocyanate ("TDI"), TDI urea, butylated hydroxy toluene ("BHT") (an antioxidant), amines, and other organic vapors. It has been found that emissions of fine particulate matter and vapors cease within about the first hour after polyurethane foam is initially manufactured and within approximately twenty minutes after the completion of other fabrication processing involving high temperatures, such as flame lamination.
While it is recognized that such materials can present environmental dangers, the removal of the fine particles and vapors from the process exhaust air stream in which they are collected is difficult due to their dilution, to the high velocity of the exhaust air stream--which is typically 6,000 to 20,000 feet per minute--and the small size of the particles and vapors.
Various methods for removing matter from gaseous streams have been disclosed in the art.
U.S. Pat. No. 3,861,889 discloses a process for removing soluble solid dust particles from a gas stream by spraying the gas stream with a solvent in which the dust is soluble and then passing the gas stream through a foamed material which is impregnated with the solvent. The patent further discloses passing the gas stream through foam filters at relatively low velocities, preferably less than 2 meters per second.
U.S. Pat. No. 3,266,228 discloses a device for separating and removing entrained liquid from air through the use of an open-cell foam material such as a polyurethane foam.
U.S. Pat. No. 4,290,783 discloses a process for removing soluble particles, specifically urea, from an air stream by first spraying the stream with water and then passing the stream through a demister.
U.S. Pat. No. 3,250,059 discloses a method for removing solid aerosols, specifically submicron particles of TiO.sub.2, from a gas stream by passing the gas stream through two sets of filters. A liquid in which the solid aerosols are insoluble is sprayed onto the first filter through a nozzle aimed in a direction concurrent to the flow of the gas stream. The liquid is employed to ensure that the solid aerosols are entrapped in the first filter.
U.S. Pat. 3,785,127 discloses a device for scrubbing a stream of air with a scrubbing liquid, e.g., water passing a gas stream through a series of screens while spraying the screens with a scrubbing liquid to facilitate the removal of small particles of minus 20 microns.
U.S. Pat. No. 2,057,579 discloses an apparatus for purifying air comprising a chamber in which baffles deflect the air so that it takes a tortuous path through the filter chamber. Liquid is sprayed against the baffle plates to remove from the flowing air relatively heavy foreign matter.
U.S. Pat. No. 3,802,158 discloses a pollution control apparatus for removing grease and smoke from an air stream. The air stream first passes through a water spray to remove large particles from the stream, following which the stream impinges upon baffles and a filter system which separate the water from the air. The filter system comprises generally, a glass wool or batting central section. Following the baffles, the stream passes through an electrostatic precipitator which removes small pollutants from the stream.
None of the methods or apparatus disclosed in these references is suitable for treating a large volume of hot and rapidly moving exhaust air stream that contains water-insoluble particulate matter and vapors emitted during the high temperature processing of polymeric materials, such as flexible polyurethane foam, in an economical manner and without special solvents.